Add --run-again option to upgrades

The --run-again option will ignore the existance of upgrade files
in /var/lib/vdsm/upgrade/

Each upgrade will still only run if it thinks it should run.

The --run-again option has two use cases:
1) It's easier to tell users to run an upgrade again with
   --run-again, then it is to tell him/her to delete some random
   file first, and then run the upgrade again. This will make
   life slightly easier when tickets start popping up.
2) During development of upgrades

Change-Id: I2e6b84bdd14b9cd3921cea187b2654be22989334
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22750
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>